Luogu P2519 [HAOI2011]problem a
\(DP\)神题。以后要多学习一个,练一练智商。
关键点在于把“有\(a_i\)个人分数比我高,\(b_i\)个人分数比我低”这句话转换成“排名为\(a_i+1\),且有\(n-a_i-b_i\)个人和我分数相同“。解决了这一点,问题就解决了一大半,接下来就变成了最大不相交区间集合选择问题。本来我是用最长路写的,不知道为什么出锅了,所以就改用\(DP\)+二分了。
#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
struct Segment {
int l, r, sz;
Segment (int _l = 0, int _r = 0, int _sz = 0) {
l = _l, r = _r, sz = _sz;
}
bool operator < (Segment rhs) const {return l == rhs.l ? r < rhs.r : l < rhs.l;}
bool operator == (Segment rhs) const {return l == rhs.l && r == rhs.r;}
} arr[N];
map <Segment, int> mp;
int n, a[N], b[N], tot, dp[N], dis[N], vis[N];
bool cmp (Segment lhs, Segment rhs) {return lhs.r == rhs.r ? lhs.l < rhs.l : lhs.r < rhs.r;}
int main () {
cin >> n;
for (int i = 1; i <= n; ++i) {
cin >> a[i] >> b[i];
if (a[i] + b[i] <= n - 1) {
arr[++tot] = Segment (a[i] + 1, n - b[i], n - a[i] - b[i]);
mp[arr[tot]]++;
}
}
sort (arr + 1, arr + 1 + tot);
tot = unique (arr + 1, arr + 1 + tot) - arr - 1;
for (int i = 1; i <= tot; ++i) {
mp[arr[i]] = min (mp[arr[i]], arr[i].sz);
// printf ("Segment %d = [%d, %d]\n", i, arr[i].l, arr[i].r);
}
sort (arr + 1, arr + 1 + tot, cmp);
for (int i = 1; i <= tot; ++i) {
dp[i] = max (dp[i], dp[i - 1]);
int l = 1, r = i;
while (l < r) {
int mid = (l + r + 1) >> 1;
if (arr[mid].r < arr[i].l) {
l = mid;
} else {
r = mid - 1;
}
}
dp[i] = max (dp[i], dp[r] + mp[arr[i]]);
}
cout << n - dp[tot] << endl;
}